<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box1
        {
            border: 10px red solid;
            /* overflow: hidden; */
          }
        .box2{
            width: 100px;
            height: 100px;
            background-color: #bfc;
            float: left;
        }
       .box1::after{
        content:'';
        display:block;
        clear: both;
       }
      
    </style>
</head>    
<body>
    <div class="box1">
        <div class="box2"></div>
        <div class="box3">aa</div>
    </div>
</body>
</html>